This invention relates to a wheel axle adjustment assembly suitable for use in vehicles that include a rigid or beam-type drive axle assembly supported by a compound leaf spring assembly. Commonly a bearing means is interposed between the axle assembly and the spring assembly to assist in mounting the axle assembly to the spring assembly.
The bearing means is usually positively located on the spring assembly by a suitable locating means, such as a cooperating spigot and socket. The positive locating means minimizes relative lateral and longitudinal movements between the axle assembly and the spring assembly.
This design works well when all the relevant components are manufactured to the close tolerances required. However, this degree of accuracy is not always achievable due to either design limitations, manufacturing limitations, or operational limitations. The end result of these limitations is that the operating performance of the relevant components is compromised.
In vehicle suspensions, for example, where at least one beam axle is located by a compound leaf spring or a semi-elliptic leaf spring, the axle(s) is designed to be located such that the longitudinal axis of the axle is perpendicular to the longitudinal axis of the vehicle's chassis. However, when manufacturing and assembling tolerances are included, an angular tolerance of .+-.0.25.degree. from perpendicular is permitted by most vehicle manufacturers. When dual rear axle suspensions are used, the pair of axles are ideally located parallel to each other, but again, a manufacturing tolerance is allowed, with a taper of .+-.1.5 mm/m being permitted. This taper results in the pair of drive wheels on one side of the vehicle being located closer together than the pair of drive wheels on the opposite side of the vehicle. These tolerances represent at best, a compromise between cost of manufacture and optimum vehicle performance.
Referring specifically to long-haul heavy vehicles, such as semi-trailer prime movers, a significant proportion of the operating cost is due to replacement of worn tires on the driving wheels. The best method of reducing this tire wear is to correctly align the drive axles. For vehicles driving on the left-hand side of the road, a suitable angular alignment of the drive axle is within the range from perpendicular to the longitudinal axis of the vehicle to 0.11.degree. rotated to the left hand side of the vehicle. This enables the drive axle of the vehicle to effectively "steer" the vehicle up the camber of the road surface. For dual axle vehicles, the axles should be further aligned with respect to each other to within the range of parallel to a taper of 0.3 mm/m towards the right-hand side of the vehicle. This taper causes the pair of drive wheels on the driver's side of the vehicle to be closer together than the pair of drive wheels on the passenger's side of the vehicle.
The suitable or desired tolerances referred to above have additional benefits other than just decreased tire wear. In the power train, for example, the tighter tolerances lead to reduced stresses and distortions in the drive shafts and mountings, and hence increase component life and decrease maintenance cost. Also, the internal stresses in bearings, caused by misalignment of components are reduced, leading to further savings in maintenance costs.
To achieve the desired tolerances, it is sometimes necessary to relocate the mounting brackets for suspension components, or in extreme cases, it may be necessary to replace or remanufacture the offending components. Inevitably this type of rework is costly, time-consuming and more often than not fails to completely solve the problem. Instead, it can exacerbate existing problems, or create a new set of problems.
Alternatively, the desired tolerances can be achieved by relocating the bearing means which position the beam axle on the semi-elliptic leaf springs. This can be accomplished by repositioning the locating means used to locate the position of the bearing means. Where the locating means is a cooperating socket and spigot, the socket in either the spring pack or the rigid plate of the insulator would be repositioned. Again, this can be a time consuming and costly exercise.
In some instances, the vehicle manufacturers have recognized the alignment problem, and have manufactured bearing means with their locating means moved from their design position by a discrete distance to cope with a degree of misalignment. These modified bearing means reduce the costs of alignment, but are only applicable for a narrow range of misalignments. If the degree of misalignment is outside the scope of the modified bearing means then the problem is merely reduced by using them, but desired tolerances cannot be achieved without performing some sort of structural modification to either the vehicle or the axle assembly.